allow dart:* libs to implement otherwise disallowed types (e.g. int)

all current and experimental Dart implementations (dart2js, vm, fletch, ddc) get an error in their SDK from this. The CL fixes it by allowing dart:* libraries to implement the disallowed types, like int/double/String/bool.

[email protected]

Review URL: https://codereview.chromium.org//1346923007 .

Author: John Messerly

pkg/analyzer/lib/src/generated/error_verifier.dart

@@ -3199,6 +3199,11 @@ class ErrorVerifier extends RecursiveAstVisitor<Object> {
     if (typeName.isSynthetic) {
       return false;
     }
+    // The SDK implementation may implement disallowed types. For example,
+    // JSNumber in dart2js and _Smi in Dart VM both implement int.
+    if (_currentLibrary.source.isInSystemLibrary) {
+      return false;
+    }
     DartType superType = typeName.type;
     for (InterfaceType disallowedType
         in _DISALLOWED_TYPES_TO_EXTEND_OR_IMPLEMENT) {